Assessing Cardiovascular Function Using an Optical Sensor

ABSTRACT

This document describes assessing cardiovascular function using an optical sensor, such as through sensing relevant hemodynamics understood by pulse transit times, blood pressures, pulse-wave velocities, and, in more breadth, ballistocardiograms and pressure-volume loops. The techniques disclosed in this document use various optical sensors to sense hemodynamics, such as skin color and skin and other organ displacement. These optical sensors require little if any risk to the patient and are simple and easy for the patient to use.

RELATED APPLICATIONS

This Application is a divisional of U.S. patent application Ser. No.14/681,625, filed Apr. 8, 2015, entitled “Assessing CardiovascularFunction Using an Optical Sensor,” the entire disclosure of which ishereby incorporated by reference.

BACKGROUND

Cardiovascular disease is the leading cause of morbidity and mortalityworldwide. At the same time this chronic disease is largely preventable.Medical science knows how to save most of these lives by removing themajor risk factors of smoking, diabetes, and hypertension. And manypeople are told just what they need to do to reduce these riskfactors—stop smoking, reduce sugar intake, eat healthier, reduce alcoholintake, increase cardiovascular exercise, lose weight, and, if needed,take blood-pressure medication. But many people do not follow this goodadvice. Because of this, millions of people needlessly die fromcardiovascular disease.

People don't follow this good medical advice because they think they aredifferent, they do not want to change their behaviors that are causingthe disease, or they do not know what to change in their particularcase. When a physician tells them that they are at risk from heartdisease because they are overweight, for example, many people know thatthis judgment is not necessarily specific to them—it is based onaverages and demographics. So being a particular weight may notnegatively affect a particular patient's heart. Further, a lack offeedback that their behavior is harming their heart results in a lack ofincentive for them to change their behavior.

This lack of incentive to follow good advice can be addressed bymonitoring the state of the patient's cardiovascular system over time toshow trends in heart health. Hard data often motivates patients tomodify their behavior, such as data indicating that their heart showsmeasurable signs of heart disease. Unfortunately, current methods formeasuring heart health can be inconvenient, stressful, and expensive.Simple home monitor products exist for measuring heart rate and bloodpressure, but long-term user compliance is a problem due toinconvenience. More advanced cardiovascular monitoring, such as heartrate variability, arterial stiffness, cardiac output, and atrialfibrillation, involve expensive and time-consuming trips to a medicalfacility for a skilled assessment. Because of this, only patients thatdemonstrate late stage symptoms of heart disease are likely to receivethese tests, which is generally too late to make simple lifestylechanges that would avoid a chronic disease.

Another reason that people don't follow this good advice, or don'tfollow it for long enough to prevent heart disease, is because they donot see the benefit. When people take the advice of changing their dietand habits—which most people do not want to do—they often don't see theimprovement before they lose the motivation to continue monitoring theircardiovascular status. Because of this, many people go back to their oldhabits only to later die of heart disease.

SUMMARY

This document describes assessing cardiovascular function using anoptical sensor, such as through sensing relevant hemodynamics understoodby heart and respiration rates, heart rate variability, blood pressures,pulse-wave velocities, arterial stiffness, cardiac valve timing,ballistocardiogram force, photo-plethysmograms, blood oxygenation, andpressure-volume loops. The techniques disclosed in this document usevarious optical sensors to sense the effects of cardiovascularhemodynamics, such as skin color or displacement at multiple spatiallocations on the body. These optical sensors require little if any riskto the patient and are simple and easy for the patient to use.

Further, the techniques described herein can determine blood flowasymmetries, which may indicate a stroke or other cardiovascular diseaseor pressure waveforms, which may indicate cardiac abnormalities, such asatrial fibrillation. These techniques may also determine trends in apatient's cardiovascular health. These trends can aid a patient byhelping them know if the effort they are expending to improve theirheart health is actually making a difference. Further, negative trendsor conditions, such as cardiac irregularities or some asymmetries can befound that can spur people to improve their health or to get medicalattention. By so doing, these techniques may save many people from dyingof heart disease.

This summary is provided to introduce simplified concepts concerning thetechniques, which are further described below in the DetailedDescription. This summary is not intended to identify essential featuresof the claimed subject matter, nor is it intended for use in determiningthe scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of techniques and devices for assessing cardiovascularfunction using an optical sensor are described with reference to thefollowing drawings. The same numbers are used throughout the drawings toreference like features and components:

FIG. 1 illustrates an example environment in which the techniques can beimplemented.

FIG. 2 illustrates an example computing device of FIG. 1.

FIG. 3 illustrates an example optical sensor of FIG. 1.

FIG. 4 illustrates a method for assessing cardiovascular function usingan optical sensor, including determination of a pulse-wave velocity fora patient.

FIG. 5 illustrates a male patient having various regions of which imagesare captured by optical sensors.

FIG. 6 illustrates various circulatory distances that can be used, alongwith time correlations, to determine a pulse-wave velocity.

FIG. 7 illustrates a method for determining circulatory distances, suchas those described in FIG. 6.

FIG. 8 illustrates a method for assessing cardiovascular function usingan optical sensor based on size, volume, or location of an organ orstructure of a patient.

FIG. 9 illustrates an example device embodying, or in which techniquesmay be implemented that assess cardiovascular function using an opticalsensor.

DETAILED DESCRIPTION Overview

This document describes techniques using, and devices enabling,assessment of cardiovascular function using an optical sensor. Throughuse of optical sensors a patient's skin color and displacement over timecan be accurately measured, including by comparing colors anddisplacements at different regions of the patient. For example, anoptical sensor can measure a color change at a patient's cheek and,based on that color change, the techniques can determine that thepatient's heart beat has produced a peak blood-pressure flow at someparticular instant at the cheek. Another optical sensor can measure acolor change or displacement at the patient's wrist for the sameheartbeat, which the techniques can determine indicates a peakblood-pressure flow at the wrist at some other instant. By comparing thetimes and distance between these regions, a pulse-wave velocity can beassessed. This pulse-wave velocity can then be used to determinearterial stiffness, blood pressure, and other measurements ofcardiovascular function. Simultaneously, those two measurement pointscan be used to independently measure other vitals like heart rate andrespiration rate, with the combination of the two used to improve themeasurement by compensating for movements, illumination changes, orocclusions.

In addition to assessing cardiovascular heath at some snapshot in time,the techniques may also measure trends in cardiovascular function. Byway of one example, assume that a patient has an optical sensor in herbathroom that is capable of measuring color and displacement at multipleregions, such as her neck, palm, and forehead. This optical sensormeasures skin color variations between or within a region, which canindicate differential blood volume to provide a photo-plethysmogram(PPG). If the patient has other optical sensors, such as one in hercomputing spectacles and another in her smartphone, these can furtheraid the accuracy and robustness of the measurements. Using thesesensors, assume that over the course of a new diet and exercise routinethat the techniques, using data from the optical sensors, determine thather heart's stroke volume (an important measure of heart health) hasimproved 6% in four weeks. With this positive feedback, this patient maycontinue her diet and exercise routine, thereby likely reducing thechances that she will die of heart disease.

For another case, assume that the techniques determine that there is anasymmetry in blood flow within a patient's face. This asymmetry can beindicated to the patient or a medical professional sufficient to performfurther testing, as asymmetry can indicate a stroke (a deadly diseasethat, with a fast diagnosis and treatment can save the patient's life orquality of life) or other vascular disease.

These are but a few examples in which assessing cardiovascular functionusing an optical sensor can be performed, other examples and details areprovided below. This document now turns to an example environment, afterwhich example optical sensors and methods, cardiovascular functions andtrends, and an example computing system are described.

Example Environment

FIG. 1 is an illustration of an example environment 100 in whichassessing cardiovascular function using an optical sensor can beemployed. Environment 100 illustrates a patient 102 that is a subject ofthe health monitoring, as well as a medical professional 104, familymember, or other caretaker that, in some cases, will receive results ofthe health monitoring. This example employs optical sensors 106, a colorand displacement optical sensor 106-1 (sensor 106-1), which is part ofcomputing device 108, and a hyperspectral sensor 106-2 (sensor 106-2),which is located within mirror 110.

Sensor data 112 is provided by each of optical sensors 106 to somecomputing device. As shown, sensor data 112 is passed from sensor 106-2to computing device 108 while sensor 106-1 is integral with computingdevice 108 and need not be passed if the techniques are performed atthat device. Computing device 108 then performs some or all of thetechniques, or passes that sensor data to some other computing device,such as a remote server through a communication network (not shown).

As shown with this example environment 100, a sensing milieu (e.g.,optical sensors 106 in patient 102's bathroom) in which a patient livescan be used that are capable of determining a cardiovascular function ofa human cardiovascular system. This sensing milieu is capable ofnon-invasively and remotely determining this cardiovascular function andtrends in this cardiovascular function. This sensing milieu sensesvarious regions of the patient, which can then be compared, timecorrelated, aggregated, averaged, and so forth to determine acardiovascular function. These cardiovascular functions can berepresented by cardiovascular asymmetries (e.g., due to a stoke),cardiac irregularities (e.g. atrial fibrillation), blood pressure,pulse-wave velocity, waveforms of circulating blood,photo-plethysmograms (PPG), ballistocardiograms, and pressure-volumeloops, to name a few.

With regard to the example computing device 108 of FIG. 1, consider adetailed illustration in FIG. 2. Computing device 108 can be one or acombination of various devices, here illustrated with seven examples: asmartphone 108-1, a server 108-2, a computing watch 108-3, computingspectacles 108-4, a laptop 108-5, a tablet computer 108-6, and a desktop108-7, though other computing devices and systems, such as a netbook orset-top box may also be used. As noted above, in some embodiments thetechniques operate, in whole or in part, through a remote device such asserver 108-2. In such cases, some computing can be forgone locally,e.g., through a communication device having limited computing operationsor even directly from optical sensors 106 to server 108-2.

Computing device 108 includes or is able to communicate with a display202 (six are shown in FIG. 2), a transceiver 204, one or more processors206, and computer-readable storage media 208 (CRM 208). Transceiver 204is capable of sending and receiving data directly or through acommunication network, such as sensor data 112 from optical sensors 106through a local area, wide area, functional area, cellular, ornear-field network.

CRM 208 includes cardiovascular-function module 210, which includes orhas access to sensor data 112 from one or more of multiple opticalsensors 106. This sensor data 112 can be associated with particulartimes 212, such that simultaneously received sensor data 112 can becorrelated to determine cardiovascular functions 214 of humancardiovascular systems and trends 216 can be determined based on sensordata 112 changing over time. CRM 208 also includes or has access to auser interface 218, that, while not required, can be used to presentdetermined trends, health, and medical advice to patient 102.

Generally, cardiovascular-function module 210 is capable of determining,based on sensor data 112, a cardiovascular function of a cardiovascularsystem of a patient, such as patient 102 of FIG. 1. With thiscardiovascular function, cardiovascular-function module 210 may alertpatient 102 or medical professionals 104 or family members/caretakers ofa negative health condition needing immediate care, for example. Medicalprofessional 104, or a specialized machine intelligence, could schedulean in-person appointment or remotely adjust patient care through changesin medication or lifestyle. Cardiovascular-function module 210 is alsoconfigured to determine trends based on the current cardiovascularfunction and prior-determined cardiovascular functions, such as thosedetermined at prior times.

More specifically, cardiovascular-function module 210 is capable ofreceiving and using optical sensor data indicating a skin, organ, orstructure's color or displacement. This data may come from single ormultiple optical sensors covering the same or different wavelengthsobserving multiple locations on the patient's body. With this data,cardiovascular-function module 210 can determine timing relationships,pulse pressure waveforms, and asymmetries in a patient's cardiovascularsystem. With this data and a circulatory distance between data fromdifferent regions of the patient, as well as time correlations betweenthe data, cardiovascular-function module 210 can determine a pulse-wavevelocity and various simple or highly sophisticated measures ofcardiovascular function, including charts of blood pressure, aballistocardiogram, a photo-plethysmogram (PPG), and pressure-volumeloops. Capabilities of cardiovascular-function module 210 are addressedfurther in methods described below.

With regard to optical sensors 106, two examples of which are shown inFIG. 1, consider a detailed illustration in FIG. 3. Generally, opticalsensors 106 are capable of detecting blood volume, color, and/ordisplacement at one or more regions of a patient. Optical sensors 106may include a standard RGB (red, green, blue) sensor, a monochromesensor, a hyperspectral sensor, a stereoscopic sensor, a structuredlight sensor, or combinations of multiple sensors, along with acombination of illumination sources such as uniform, infrared,tangential, modulated/coded, or coherent (laser). Optical sensors 106may also have a fixed camera position or consist of one or more camerasand light sources on mechanical targeting platforms or those that simplymove due to being part of a mobile device. Optical sensors 106 may alsobe separated into physically and spatially distinct devices capable ofmonitoring the body from multiple view angles or observing differentregions of the body. Thus, one of optical sensors 106 may capture animage indicating blood volume at two different regions of patient 102,which then can be compared, by cardiovascular-function module 210, todetermine a blood-volume asymmetry or other cardiac function. In thecase of a blood-volume asymmetry, a difference in vascular functionbetween the regions may indicate a cardiac-related health problem, suchas a stroke. Optical sensors 106 provide various types of information,and are not limited to determining asymmetries.

In more detail, optical sensor 106 can be one or a combination ofvarious devices, here illustrated with color and displacement opticalsensor 106-1 (e.g., a camera of computing device 108), sensor 106-2,which is stationary and located within mirror 110, a wearable color anddisplacement optical sensor 106-3, which is part of computing spectacles108-4, structured-light or stereoscopic sensor system 106-4, and opticsensor 106-5 of laptop 108-5. The cameras can also be motorized toaccurately point at specific points on the body.

As noted in part, sensor 106-2 is capable of capturing images in anultraviolet, visible, or infrared optical wavelength. Images recordingthese wavelengths can be used to determine various changes in bloodmovement or as calibration signals to detect changes in illumination orpatient movement. In some cases blood perfusion and oxygen content canbe ascertained, thereby further enabling robust measurement of cardiacfunction. Due to differential wavelength absorption between human tissueand blood, a hyperspectral sensor can also be used to penetrate the skinto map out veins and arteries to target closer examination fordisplacement and other measurements.

Structured-light sensor system 106-4 is capable of projecting structuredlight at patient 102 and sensing, often with two or more opticalsensors, the projected structured light on patient 102 effective toenable capture of images having surface information. This surfaceinformation can be used to calculate depth and surface changes for aregion of patient 102, such as skin, another organ, or other structure.These changes can be highly accurate, thereby indicating smallvibrations and other changes in an organ or structure caused by thecardiovascular system, and thus how that system is operating.Structured-light sensor system 106-4 can, alternatively, be replacedwith or supplemented with a targeted, coherent light source formore-accurate displacement measurements. This may include LIDAR (e.g.,“light radar” or the process measuring distance by illuminating a targetwith a laser and analyzing light reflected from the target), laserinterferometry, or a process of analyzing light speckle patternsproduced by a coherent light on a skin's surface through opticaltracking, which enables detection of very small skin displacements.

These optical sensors 106 can capture images with sufficient resolutionand at sufficient shutter speeds to show detailed colors anddisplacement, and thus enable determination of mechanical movements orvibrations. These mechanical movements and mechanical vibrations aresufficient to determine a ballistocardiogram (BCG) showing patient 102'scardiac function. Other sensing manners, such as color change or skindisplacement in a different region of a patient's body, can be used toestablish motion frequency bands to amplify, as well as a timingreference for aggregating multiple heartbeat measurements to improveaccuracy of a BCG motion. This BCG information can also be used toprovide reference timing information about when a blood pressure pulseleaves the left ventricle and enters the aorta, which combined with theother measurements across the body allows for more-precise estimates ofpulse transit times and pulse-wave velocities.

While the BCG signal indicates the timing of the aortic valve, thetiming of the atrial valve can be monitored by tracking atrial pressurewaveforms visible in the external or internal jugular. This also allowsfor the opportunity to detect atrial fibrillation by detecting missingatrial-pressure pulses. Additionally, aortic-wall stiffness has provenprognostic value in predicting cardiovascular morbidity and mortality.Measuring the pulse-transit time from the start of ejection from theleft ventricle into the aorta and up the carotid allows an estimate ofthat aortic stiffness as well as trending of changes in that stiffness.Thus, determination of arterial-wall stiffness can made independent ofblood pressure measurements.

In more detail, optical sensors 106 are configured to capture sufficientinformation for the techniques to determine blood asymmetries and othercardiac function, including a pulse-wave velocity of patient 102'sblood. This pulse-wave velocity is a measure of a patient's arterialhealth. In healthy arteries the pulse-wave velocity is low due to theelasticity of the arteries but, as they harden and narrow, thepulse-wave velocity rises. Additionally, as blood pressure increases anddilates the arteries, the walls become stiffer, increasing thepulse-wave velocity. While a particular pulse-wave velocity as asnapshot in time may or may not accurately indicate cardiovascularhealth (e.g., a one-time test at a doctor's office), a change in thispulse-wave velocity (that is, a trend), can be an accurate measure of achange in patient 102's cardiovascular health. If a positive trend, thiscan reinforce patient 102's healthy habits and, if negative, encouragechanges to be made.

In more detail, each of the color-sensing optical sensors 106 isconfigured to record colors in a patient's skin sufficient to determinea photo-plethysmogram. This PPG measures variations in a size or colorof an organ, limb, or other human part from changes in an amount ofblood present in or passing through it. These colors and colorvariations in a patient's skin can show heart rate and efficiency.

These examples show some ways in which the techniques can providesubstantially more-valuable (or at least different) data by which toassess a patient's cardiac function than those provided in a medicaloffice or hospital. As noted, conventional health monitoring is oftenperformed at a hospital or medical practitioner's office. Healthmonitoring at a hospital or office, however, cannot monitor a patientduring their normal course of life or as often as desired. This can be aserious limitation because a snapshot captured at a hospital or officemay not accurately reflect the patient's health or may not performed atall due to the infrequency of a patient's visits. Even if testing at ahospital or medical office is performed often, it can be inaccurate dueto it being of a short duration or due to the testing being in anartificial environment. Note that this does not preclude the use of thetechniques disclosed herein at a hospital or medical office, where theymay prove valuable in supplementing or replacing conventionalmeasurements, and in the case of in-patient care, may provide a mannerfor continuous monitoring of patients that are critically (or otherwise)ill.

Returning to FIG. 3, optical sensor 106 generally may have variouscomputing capabilities, though it may instead be a low-capability devicehaving little or no computing capability. Here optical sensor 106includes one or more computer processors 302, computer-readable storagemedia 304, image capture element 306, and a wired or wirelesstransceiver 308 capable of receiving and transmitting information (e.g.,to computing device 108). Image capture element 306 may include simpleor complex cameras, such as those having low or high shutter speeds, lowor high frame rates, low or high resolutions, and having or not havingnon-visible imaging capabilities. Computer-readable storage media 304includes optics manager 310, which is capable of processing sensor dataand recording and transmitting sensor data, as well as receive or assignappropriate time markers by which to mark or compare the time of variouscaptured images. Optics manager 310 and cardiovascular-function module210 may also calibrate image capture element 306 through use of anexternal sensor. This can aid in calibrating skin colors ordisplacements to a calibration color or displacement, or even to acardiac function, such as to a blood pressure or pulse-wave velocity.Thus, while one of optical sensors 106 captures images for two regions,a blood pressure between those regions is also measured through adifferent device, thereby enabling more-accurate determination ofcardiac functions for the optical sensor and for that patient. Otherpotential calibration sensors include, but are not limited to, ECG,conventional BCG, digital stethoscopes, ultrasound, and the like.Another example is the use of an external blood pressure meter tocalibrate the pulse wave velocity over time to determine long-termchanges in arterial-wall stiffness by separating arterial stiffness dueto blood pressure versus that due to the dilation by blood pressure.

These and other capabilities, as well as ways in which entities of FIGS.1-3 act and interact, are set forth in greater detail below. Theseentities may be further divided, combined, and so on. The environment100 of FIG. 1 and the detailed illustrations of FIGS. 2 and 3 illustratesome of many possible environments capable of employing the describedtechniques.

Example Methods

FIGS. 4 and 8 depict methods that assess cardiovascular function usingan optical sensor. These methods are shown as sets of blocks thatspecify operations performed but are not necessarily limited to theorder or combinations shown for performing the operations by therespective blocks. In portions of the following discussion reference maybe made to environment 100 of FIG. 1 and entities detailed in FIGS. 2and 3, reference to which is made for example only. The techniques arenot limited to performance by one entity or multiple entities operatingon one device.

At 402, skin colors or skin displacements are received from one or moreoptical sensors. These skin colors or displacements are captured atregions of a patient, such as a color captured at a patient's skin onher forehead and a displacement of skin on her neck or on her clavicle.Optionally, as part of operation 402, cardiovascular-function module 210or optics manager 310 may automatically determine which regions of apatient are fully visible or partially occluded, and thereby determinebetter regions of a patient to capture images.

By way of illustration, consider FIG. 5, which shows a male patient 502having various regions 504 of which images are captured. These regions504 include, by way example, a cheek region 504-1, a neck region 504-2,an outer wrist region 504-3, an outer hand region 504-4, an inner wristregion 504-5, a palm region 504-6, a front ankle region 504-7, and aninner ankle region 504-8, to name but a few. By way of an ongoingexample, assume that one optical sensor captures a color change ordisplacement of skin at neck region 504-2 and another color change ordisplacement of skin at inner wrist region 504-5.

At 404, a circulatory distance is determined between the regions of thepatient at which the colors or displacements are captured. Thiscirculatory distance can be an approximation based on a linear distancebetween the regions, such as a linear distance based on an axialdistance oriented relative to an axis of the patient's spine, or simplya vertical distance with the patient standing. In some cases, however,the techniques determine or approximate a circulatory distance based onan arterial-path distance. This arterial-path distance can be determinedor approximated using an arterial structure of the patient or determinedbased on a skeletal structure of the patient, including automatically byoptical visualization of the patient.

By way of illustration of the various circulatory distances that can beused, consider FIG. 6. Here assume that multiple images are captured ofpatient 502's neck region 504-2 (also shown in FIG. 5) sufficient todetermine a neck waveform 602. Multiple images are also captured ofinner wrist region 504-5 sufficient to determine an inner-wrist waveform604. At operation 404, cardiovascular-function module 210 determines thecirculatory distance from neck region 504-2 and inner wrist region 504-5in one of the following four manners. In the first, a vertical distanceD_(v) is calculated with the patient standing. In the second, an axialdistance D_(axial) is calculated based on the distance relative to anaxis of the patient's spine—here it is similar to the vertical distance,D_(v), but if the person is oriented at an angle, the distances aredifferent. In the third, cardiovascular-function module 210 calculatesthe distance as a point-to-point between the regions, here shown asD_(ptp). In the fourth, cardiovascular-function module 210 calculates orapproximates the distance that blood travels through patient 502'sarteries, D_(path). This arterial-path distance can be determined basedon the arteries themselves or an approximation based on a skeletalstructure or an overall body shape of the person. Data for skeletalstructure and overall body shape can be determined using images capturedfor the regions and structures in between the regions, optically orotherwise. In some cases radar can be used that penetrates clothing totrack bony surfaces, thereby providing a skeletal structure from whicharterial distance can be approximated.

While not required, operation 404 may be performed, in whole or in part,using method 700 illustrated in FIG. 7, which is described followingmethod 400 below. By way of overview, in this example method, thetechniques determine one or more of the distances illustrated in FIG. 6.

The more-accurate distance calculations provide a better pulse-wavevelocity, and thus indicate a current cardiovascular function. Whilepotentially valuable, more-accurate distances are not necessarilyrequired to show trends in cardiovascular function. Trends are providedby consistently calculated distances more than accurate distances, andfor a specific individual, should not change significantly over time forsame measurement points. If the measurement points vary due tovisibility issues (such as clothing), then distance measurementestimates increase in importance for accurate trending.

At 406, a time correlation between capture of the colors anddisplacements is determined. This time correlation is between theinstant of capture at the regions, as this time correlation is laterused. Cardiovascular-function module 210 may determine the timecorrelation based on a time at which a maximum or minimum blood volumeis determined for each of the regions, or some other consistent andcomparable point in a waveform, such as a beginning of a pressureincrease in the waveform (show in FIG. 6). In more detail, this timecorrelation can be considered a temporal distance between multipleimages capturing some measure of cardiac operation, such as blood volumeat each of the regions. Thus, by comparing various images for a regioncardiovascular-function module 210 can determine a maximum, minimum, ormedian color at the region as well as at another region, and bycomparing these and times at which each were taken, can determine thetime correlation for a same heartbeat.

Note that waveforms 602 and 604 can be determined through color, or insome locations of the body, related waveforms can be determined throughdisplacement. Cardiovascular-function module 210 can determine, based ona change in color to regions over time, a waveform. These color changesindicate a peak or crest of a wave based on blood content at the organand thus can be used to determine a shape of the wave. While a shape ofa wave can differ at different regions, they can still be compared tofind a time correlation. In the case of lower-than-desired optical framerates due to sensitivity or processing limitations, interpolation orcurve fitting can be used to improve the estimate of the waveform forimproved time correlation. Repeated measurements, which are time shiftedrelative to the pressure wave either naturally by the optical samplingfrequency or intentionally by adjusting the sampling frequency, canbuild up a super-sampled estimate of the waveform. The highertiming-resolution waveform can be used for more-accurate timingmeasurements. Additionally, displacements, either through directdistance measurements or tangential shading, can show signals related tothe pressure waveforms as the arteries and veins expand and contract.These waveforms can further reveal cardiac activity, such as valvetiming, valve leakage (regurgitation), fibrillation, stroke volume, andthe like.

At 408, a pulse-wave velocity for blood circulation through the patientis determined based on the circulatory distance and the timecorrelation, as well as the skin colors or displacements. As shown inFIG. 6, the time correlation is based on similar points in a waveformand the circulatory distance is some calculation or approximation of thedistance blood travels from regions at which images are captured. Inmore detail, a pulse-wave velocity is the circulatory distance dividedby the time correlation.

Pulse-wave velocity is a good measure of cardiac function. It canindicate, for example, an arterial stiffness of a patient (the fasterthe pulse-wave velocity, the higher the arterial stiffness), a bloodpressure, and a mean arterial pressure for the patient. In more detail,the techniques can determine blood pressure based on the pulse-wavevelocity using the Bramwell-Hill equation, which links pulse-wavevelocity to compliance, blood mass density, and diastolic volume. Eachof these are measures of cardiac function that can indicate a patient'scardiac health. As noted above, the techniques can provide these cardiacfunctions to a patient, thereby encouraging the patient to make changesor, in some cases, seek immediate medical care.

Note that, in some cases, three or more different regions are measuredat operation 402. In these cases, cardiovascular-function module 210 maydetermine which of the regions are superior to others, such as due todata captured for those regions being noisy or incomplete or otherwiseof inferior quality. Those that are superior can be used and the othersdiscarded, or cardiovascular-function module 210 may weigh thedetermined pulse wave velocity between different regions based on thequality of the data used to determine those pulse wave velocities. Thiscan be performed prior to or after recording those pulse wave velocitiesas described below.

Following determination of the pulse-wave velocity at operation 408, thetechniques may proceed to record the pulse-wave velocity at operation410 and the repeat operations 402-410 sufficient to determine a trend atoperation 412. In some cases, however, the determined pulse-wavevelocity is provided, at operation 414, to the patient or medicalprofessional. Optionally, calibration data from an external sensor canbe used to improve performance. For example, an external blood pressuremonitor could be used to train the system to correlate PWV with bloodpressure. The device could be captured through an electronic network(Bluetooth™ or the like) or the optical system could scan the userinterface and perform OCR to read the results. Machine learning could beapplied to create a patient specific model for estimating blood pressurefrom PWV.

At 412, a cardiovascular trend for the patient is determined based onmultiple pulse-wave velocity measurements, such as comparing prior andlater-time determined pulse-wave velocities. This can simply show atrend of pulse-wave velocities rising or falling, such as with velocityrising due to increased arterial stiffness. Multiple locations acrossthe body can be measured to map changes over time.Cardiovascular-function module 210 may also determine other measures ofcardiac function, such as changes in flow asymmetries or pulse pressurewaveforms over time.

At 414, as noted, this trend determined at operation 412, or apulse-wave velocity determined at operation 408, is provided to thepatient or a medical professionals, e.g., patient 102 or 600 and medicalprofessional 104, of FIG. 1 or 6.

In some cases skin color, skin displacement, or both are used by thetechniques in method 400. Thus, color changes can indicate blood flowover time, as can displacement changes. Furthermore, use of color anddisplacement both can indicate an amount of blood in capillaries in theskin while displacement can indicate a change to a volume of the skin oran organ under the skin, such as vein or artery, and thus an amount ofblood in the skin or near it can be determined.

Note also that the techniques may repeat operations of method 400 forvarious other regions. Doing so may aid in altering the pulse-wavevelocity to improve its accuracy or robustness by determining anotherpulse-wave velocity between two other regions or between another regionand one of the regions for which images are captured. Thus, thetechniques may determine a pulse-wave velocity for the patient based ontwo pulse-wave velocities between regions, such as regions 504-3 and504-1, 504-7 and 504-1, and/or 504-8 and 504-2.

As noted above, method 400 can be supplemented, and operation 404 may beperformed, in whole or in part, using method 700 illustrated in FIG. 7.In this example method, the techniques determine one or more of thedistances illustrated in FIG. 6. For operations 702-706, a patient'scirculatory distances between regions are establish for later use as amanner in which to calibrate the patient's distances. While calibrationfor a single sensing milieu to determined trends may not be required,use of different sensing milieus or to determine a cardiovascularfunction with quantitative precision both aid from use of calibration.Operation 708 and 710 can be used as one way in which the techniques mayperform operation 404 of method 400.

At 702, a distance between various regions is measured, optically,manually, or in other manners. Consider, for example, capturing an imageof patient 502 of FIG. 5. Assume that some physical data is available,such as a distance between the optical sensor capturing the image andpatient 502, or a height of patient 502, and so forth. With thisphysical data, the distance can be determined from the image. Generally,this distance is from point-to-point, and is later analyzed forcirculatory distance. Other manners can also or instead be used, such asa nurse measuring patient 502, either from point-to-point or alongstructures, such as from a wrist to an elbow, elbow to shoulder, andfrom shoulder to heart. A patient may also interact with optical sensor106 and cardiovascular-function module 210 to calibrate distancesbetween regions, such as standing at a particular location relative tooptical sensor 106 and so forth. Various other technologies can be usedas well, such as structured light optical sensors, radar, LIDAR, andSODAR (measuring distance through use of sound through air).

At 704, a circulatory distance is determined using the measureddistance. In some cases the measured distance is simply used as thecirculatory distance, such as measuring D_(ptp) and then using D_(ptp)(of FIG. 6) as the circulatory distance. As noted in part herein,however, other circulatory distances may be determined, such asmeasuring a point-to-point where patient 502's arm is bent, and thuscalculating a fully extended point-to-point to maintain consistency ofcirculatory distance. Other examples include measuring D_(v) and then,based on data about patient 502, determining an arterial-path distance(D_(path)).

At 706, these various determined circulatory distances are associatedwith the patient's identity. The identity of the patient can be entered,queried from the patient, or simply associated with some repeatablemeasure of identity, even if the person's name is not known. Examplesinclude determining identity using fingerprints or facial recognition,and then associating distances with that fingerprint or facialstructure.

At 708, the patient's identity is determined. This can be performed aspart of operation 404. With this identity, at 710 circulatory distancesbetween regions are determined. For example, cardiovascular-functionmodule 210 may use facial recognition to identify patient 502 and, afterdetermining patient 502's identity, find previously determinedcardiovascular distances between each of regions 504 by simply mappingthe relevant regions to previously stored distances. When cardiovasculartime correlations are determined at operation 406, a pulse wave velocitycan be determined using the mapped-to cardiovascular distance for theregions measured.

FIG. 8 depicts a method for assessing cardiovascular function using anoptical sensor based on size, volume, or location of an organ orstructure of a patient. In method 800, images are captured over 2 to 10millisecond-range or faster timeframes, thereby providing multipleimages relating to an organ or structure of the patient. Note thatsub-millisecond timeframes can also be useful for measure acousticvibrations and are optional. Method 800 may operate, in whole or inpart, in conjunction with method 400, though this is not required.

At 802, structured light is projected onto an organ or structure of apatient. Note that this is optional, though in some cases use ofstructured light aids in accurate measurement of movement anddisplacement of a region of the patient. Alternatively, tangential lightmay be used to generate shadowing to detect skin displacement, or acoded light source could be used to reject external interference. Forexample, an alternating on and off light source at the frame rate wouldallow sampling and canceling of the background illumination. Further,light reflected from background objects or patient clothing can be usedto track changes in lighting over time or in different conditions, e.g.,daylight vs night, light bulb luminosity degradation over time, and soforth. With this data, ambient light and its effect on images capturedcan be calibrated and for which cardiovascular-function module 210 canadjust for the various methods described herein.

At 804, multiple images are received that capture an organ or structureof a patient. As noted, the images captured may include capture ofstructured light to aid in determining displacement using surfaceinformation captured. This surface information can be from one ormultiple devices. These multiple images can be received from one ormultiple optical sensors and over various timeframes, such as thosecaptured at millisecond-range or faster timeframes.

At 806, changes in the size, volume, or location of the organ orstructure of the patient are determined. These changes are determined bycomparing sizes, volumes, or locations of the organ or structure of thepatient recorded by the various multiple images captured over time. Notethat these changes can be used in coordination with, or to compensatefor, data from methods 400, and vice-versa. Thus, data from one portionof the body captured in any of the various manners described herein, canbe used to compensate for other data, such as using a color or waveformdetermined at method 400 to compensate for motion artifacts in the dataof method 800.

At 808, a cardiac function of the patient is determined based on thechanges. This cardiac function can be one of the many described above,including heart rate, blood pressure, pulse-wave velocity, pressurevolume loops, blood-volume and other asymmetries, and so forth, as wellas respiration rate.

By way of a first example, consider a case where an asymmetry isdetermined between to different regions of the patient. In some casesthis asymmetry is determined by blood-volume differences, which can beindicated by size or color. To determine an asymmetry,cardiovascular-function module 210 may compare the differentcardiovascular pulse times of the regions, where one of the pulse timesfor a same heart beat is different, as it is further from the patient'sheart. Alternatively, the waveform's peak, median, or trough of bloodvolume can be accurately compared. Thus, assume that a right wrist and aleft wrist of a patient have different blood volumes at each of theirpeaks, with one being a lower peak blood volume that the other, therebyindicating some difference in vascular function.

Cardiac function trends, as noted in part above, can greatly aid inhelping patients maintain or change their habits to improve theircardiac health. Consider, for example, a trend showing a change to acardiovascular function over weeks, months, or years using thetechniques. This trend can show cardiac function in many ways superiorto the best invasive cardiac testing because a trend need not requireperfect accuracy—instead consistency is used. Furthermore, this can beperformed by the techniques without interrupting the patient's day,making the patient perform a test, or requiring the patient to go see amedical professional. By so doing, many lives can be saved.

In more detail, consider the techniques in the context of FIGS. 1-3.Here various kinds of optical sensors 106 sense regions (e.g., regions504 of FIG. 5) of a patient (e.g., patient 102 of FIG. 1 or patient 502of FIGS. 5 and 6) through image capture elements 306. This sensor data(e.g., images) are then processed and/or stored by optics manager 310(e.g., to mark the images with times), after which they are passed,through wired/wireless transceiver 308 as sensor data 112 tocardiovascular-function module 210 operating on computing device 108 ofFIG. 2. Also passed are indications of the region and the times 212 atwhich the sensor data 112 was captured.

Cardiovascular-function module 210 then performs operations of method400 and/or method 800 to determine cardiac function, as noted above.Consider, for example, a case where cardiovascular-function module 210determines that a cardiac function meets or exceeds a safety threshold.Example safety thresholds include a blood pressure being too high, aheart rate being too rapid or irregular, or a low blood-oxygen level.This safety threshold can also be complicated or more difficult todetermine, such as a patient's heart showing an end-diastolic volumeejected out of a ventricle during a contraction being less than 0.55(this is a measure of ejection fraction (EF) and low fractions canindicate a heart attack is imminent). These are but a few of the manysafety thresholds for cardiac function enabled by the techniques. If asafety threshold is exceeded, medical professional 104 (orfamily/caretaker) and patient 102 can be informed, such by operation 810of method 800.

The preceding discussion describes methods relating to assessingcardiovascular function using an optical sensor for a humancardiovascular system. Aspects of these methods may be implemented inhardware (e.g., fixed logic circuitry), firmware, software, manualprocessing, or any combination thereof. These techniques may be embodiedon one or more of the entities shown in FIGS. 1-3 and 9 (computingsystem 900 is described in FIG. 9 below), which may be further divided,combined, and so on. Thus, these figures illustrate some of the manypossible systems or apparatuses capable of employing the describedtechniques. The entities of these figures generally represent software,firmware, hardware, whole devices or networks, or a combination thereof.

Example Computing System

FIG. 9 illustrates various components of example computing system 900that can be implemented as any type of client, server, and/or computingdevice as described with reference to the previous FIGS. 1-8 toimplement techniques for assessing cardiovascular function using anoptical sensor. In embodiments, computing system 900 can be implementedas one or a combination of a wired and/or wireless wearable device,System-on-Chip (SoC), and/or as another type of device or portionthereof. Computing system 900 may also be associated with a user (e.g.,a patient) and/or an entity that operates the device such that a devicedescribes logical devices that include users, software, firmware, and/ora combination of devices.

Computing system 900 includes communication devices 902 that enablewired and/or wireless communication of device data 904 (e.g., receiveddata, data that is being received, data scheduled for broadcast, datapackets of the data, etc.). Device data 904 or other device content caninclude configuration settings of the device, media content stored onthe device, and/or information associated with a user of the device.Media content stored on computing system 900 can include any type ofaudio, video, and/or image data, including complex or detailed resultsof cardiac function determination. Computing system 900 includes one ormore data inputs 906 via which any type of data, media content, and/orinputs can be received, such as human utterances, user-selectable inputs(explicit or implicit), messages, music, television media content,recorded video content, and any other type of audio, video, and/or imagedata received from any content and/or data source.

Computing system 900 also includes communication interfaces 908, whichcan be implemented as any one or more of a serial and/or parallelinterface, a wireless interface, any type of network interface, a modem,and as any other type of communication interface. Communicationinterfaces 908 provide a connection and/or communication links betweencomputing system 900 and a communication network by which otherelectronic, computing, and communication devices communicate data withcomputing system 900.

Computing system 900 includes one or more processors 910 (e.g., any ofmicroprocessors, controllers, and the like), which process variouscomputer-executable instructions to control the operation of computingsystem 900 and to enable techniques for, or in which can be embodied,assessing cardiovascular function using an optical sensor. Alternativelyor in addition, computing system 900 can be implemented with any one orcombination of hardware, firmware, or fixed logic circuitry that isimplemented in connection with processing and control circuits which aregenerally identified at 912. Although not shown, computing system 900can include a system bus or data transfer system that couples thevarious components within the device. A system bus can include any oneor combination of different bus structures, such as a memory bus ormemory controller, a peripheral bus, a universal serial bus, and/or aprocessor or local bus that utilizes any of a variety of busarchitectures.

Computing system 900 also includes computer-readable media 914, such asone or more memory devices that enable persistent and/or non-transitorydata storage (i.e., in contrast to mere signal transmission), examplesof which include random access memory (RAM), non-volatile memory (e.g.,any one or more of a read-only memory (ROM), flash memory, EPROM,EEPROM, etc.), and a disk storage device. A disk storage device may beimplemented as any type of magnetic or optical storage device, such as ahard disk drive, a recordable and/or rewriteable compact disc (CD), anytype of a digital versatile disc (DVD), and the like. Computing system900 can also include a mass storage media device 916.

Computer-readable media 914 provides data storage mechanisms to storedevice data 904, as well as various device applications 918 and anyother types of information and/or data related to operational aspects ofcomputing system 900. For example, an operating system 920 can bemaintained as a computer application with computer-readable media 914and executed on processors 910. Device applications 918 may include adevice manager, such as any form of a control application, softwareapplication, signal-processing and control module, code that is nativeto a particular device, a hardware abstraction layer for a particulardevice, and so on.

Device applications 918 also include any system components, modules, ormanagers to implement the techniques. In this example, deviceapplications 918 include cardiovascular-function module 210 or opticsmanager 310.

CONCLUSION

Although embodiments of techniques for, and apparatuses enabling,assessing cardiovascular function using an optical sensor have beendescribed in language specific to features and/or methods, it is to beunderstood that the subject of the appended claims is not necessarilylimited to the specific features or methods described. Rather, thespecific features and methods are disclosed as example implementationsof these techniques.

What is claimed is:
 1. A computer-implemented method comprising:receiving, from an optical sensor capable of detecting blood volume attwo or more regions of a patient, an image indicating a first bloodvolume for a first region and another image indicating a second bloodvolume for a second region; comparing the first blood volume for thefirst region and the second blood volume for the second region toprovide a blood-volume asymmetry; and determining, based on theblood-volume asymmetry, a difference in vascular function between thefirst region and the second region.
 2. The method of claim 1, whereindetermining the difference in vascular function is based on thecomparing the first and second blood volumes indicating a lower peakblood volume at one of the first or second blood volumes than the otherof the first or second blood volumes.
 3. The method of claim 1, furthercomprising indicating, to the patient or a medical professionalassociated with the patient, a potential stroke or other vasculardisease based on the difference in vascular function.
 4. The method ofclaim 1, wherein the image and the other image indicate the first bloodvolume and the second blood volume with skin colors and furthercomprising determining the first blood volume and the second bloodvolume using the skin colors of the first and second regions,respectively.
 5. The method of claim 1, wherein the image and the otherimage indicate the first blood volume and the second blood volume withskin displacements and further comprising determining the first bloodvolume and the second blood volume using the skin displacements of thefirst and second regions, respectively.
 6. The method of claim 1,further comprising calibrating the optical sensor with one or moreexternal sensors effective to calibrate the first blood volume and thesecond blood volume to a calibration blood-volume asymmetry, thecalibrating prior to determining the blood-volume asymmetry, and whereinproviding the blood-volume asymmetry is based at least in part on thecalibration.
 7. A system comprising: an optical sensor capable ofdetecting blood volume at two or more regions of a patient; a computerprocessor; and one or more computer-readable storage media havinginstructions stored thereon that, responsive to execution by thecomputer processor, performs operations comprising: receiving, from theoptical sensor, an image indicating a first blood volume for a firstregion and another image indicating a second blood volume for a secondregion; comparing the first blood volume for the first region and thesecond blood volume for the second region to provide a blood-volumeasymmetry; and determining, based on the blood-volume asymmetry, adifference in vascular function between the first region and the secondregion.
 8. The system of claim 7, wherein determining the difference invascular function is based on the comparing the first and second bloodvolumes indicating a lower peak blood volume at one of the first orsecond blood volumes than the other of the first or second bloodvolumes.
 9. The system of claim 7, the operations further comprisingindicating, to the patient or a medical professional associated with thepatient, a potential stroke or other vascular disease based on thedifference in vascular function.
 10. The system of claim 7, wherein theimage and the other image indicate the first blood volume and the secondblood volume with skin colors and the operations further comprisedetermining the first blood volume and the second blood volume using theskin colors of the first and second regions, respectively.
 11. Thesystem of claim 7, wherein the image and the other image indicate thefirst blood volume and the second blood volume with skin displacementsand the operations further comprise determining the first blood volumeand the second blood volume using the skin displacements of the firstand second regions, respectively.
 12. The system of claim 7, theoperations further comprising calibrating the one or more opticalsensors with one or more external sensors effective to calibrate thefirst blood volume and the second blood volume to a calibrationblood-volume asymmetry, the calibrating prior to determining theblood-volume asymmetry, and wherein providing the blood-volume asymmetryis based at least in part on the calibration.
 13. The system of claim 7,the operations further comprising: performing the operations again at alater time to determine a later-time difference in vascular function;and determining a cardiac trend for the patient based on the later-timevascular difference and the vascular difference.
 14. One or morecomputer-readable storage media having instructions stored thereon that,responsive to execution by a computer processor, performs operationscomprising: receiving, from an optical sensor capable of detecting bloodvolume at two or more regions of a patient, an image indicating a firstblood volume for a first region and another image indicating a secondblood volume for a second region; comparing the first blood volume forthe first region and the second blood volume for the second region toprovide a blood-volume asymmetry; and determining, based on theblood-volume asymmetry, a difference in vascular function between thefirst region and the second region.
 15. The media of claim 14, whereindetermining the difference in vascular function is based on thecomparing the first and second blood volumes indicating a lower peakblood volume at one of the first or second blood volumes than the otherof the first or second blood volumes.
 16. The media of claim 14, whereinthe instructions further perform operations comprising indicating, tothe patient or a medical professional associated with the patient, apotential stroke or other vascular disease based on the difference invascular function.
 17. The media of claim 14, wherein the image and theother image indicate the first blood volume and the second blood volumewith skin colors and the instructions further perform operationscomprising determining the first blood volume and the second bloodvolume using the skin colors of the first and second regions,respectively.
 18. The media of claim 14, wherein the image and the otherimage indicate the first blood volume and the second blood volume withskin displacements and the instructions further perform operationscomprising determining the first blood volume and the second bloodvolume using the skin displacements of the first and second regions,respectively.
 19. The media of claim 14, wherein the instructionsfurther perform operations comprising calibrating the one or moreoptical sensors with one or more external sensors effective to calibratethe first blood volume and the second blood volume to a calibrationblood-volume asymmetry, the calibrating prior to determining theblood-volume asymmetry, and wherein providing the blood-volume asymmetryis based at least in part on the calibration.
 20. The media of claim 14,wherein the instructions further perform operations comprising:performing the operations again at a later time to determine alater-time difference in vascular function; and determining a cardiactrend for the patient based on the later-time vascular difference andthe vascular difference.